The present invention is related to brake systems that are equipped with brake lights that are turned on in response to an increase in pressure in the master cylinder which is used to pressurize the brakes. The body of the master cylinder is tapped on the high pressure side of the piston and connected to the brake. A pressure responsive light switch is also connected through a threaded opening into the high pressure side of the master cylinder. Both of these connections are made separately to the master cylinder. This requires special machining of the master cylinder body, as well as a special adaptor for connecting the cartridge to the brake line.